1. Field of the Invention
The present invention relates to a visible light wireless communication apparatus and a line coding technology for removing flickering thereof, and more particularly, to a visible light wireless communication apparatus capable of reducing flickering in a visible light wireless communication system and a line coding method for removing flickering.
2. Description of the Related Art
Visible rays are rays having a wavelength in the range visible to a human eye among electromagnetic waves. The wavelength of the visible rays corresponds to 380 nm to 780 nm.
The change in nature according to the wavelength is represented by each color within the visible rays. The wavelength is short going from red to violet. Light having a wavelength longer than red is referred to as an infrared ray and light having a wavelength shorter than violet is referred to as an ultraviolet ray. In monochromatic light, red is shown at a wavelength of 700 to 610 nm, orange is shown at a wavelength of 610 to 590 nm, yellow is shown at a wavelength of 590 to 570 nm, green is shown at a wavelength of 570 to 500 nm, and blue is shown at a wavelength of 500 to 450 nm, and violet is shown at a wavelength of 450 to 400 nm. Various colors can be represented by a mixture of colors having each wavelength.
Unlike ultraviolet ray or infrared ray, the visible ray is light visible to a person. The lighting that radiates the visible ray should meet various requirements such as accurate color representation, or the like. Small flickering is one of the requirements.
Since a human being cannot recognize flickering of 200 times or more per second, lighting using a light emitting diode (LED) having fast flickering performance control flickering using pulse width modulation (PWM) in order to expand the lifespan of the LED while saving energy.
Among communication technologies, a line coding technology as a technology for removing a DC component has been known. As an example of the well-known line coding technology, there are 4B5B, 8B10B, Manchester code, or the like. The line coding technology changes data to be transmitted into an optional pattern so that 0 or 1 is not continuously generated.
For example, the 4B5B line code changes 4-bit transmission data into 5 bits. Since 4 bits have 16 bit patterns and 5 bits have 32 bit patterns, only 16 patterns in which 1 and 0 is considerably changed among 32 patterns of 5 bits are selected and corresponds one-to-one the transmission data pattern, thereby making it possible to reduce continuous generation of 0 or 1.
Visible light wireless communication mainly uses on/off keying (OOK), that is, a type of turning-on the LED when data is 1 and a type of turning-off the LED when data is 0. In this case, when the ratio of 1 and 0 per unit time is not constant, brightness is changed, such that there is a problem in that a user sees the flickering. In other words, when transmitting data in which only 1 is repeated for 1 second and only 0 is again repeated for 1 second, the LED lighting is flickered in a unit of 1 second. Due to the problem, the line coding technology is used not to continuously generate 1 and 0.
However, when the visible light wireless communication technology including the line coding technology according to the related art is applied to the lighting using the LED, there is a problem in that it is more likely for a human being to recognize the flickering. For example, in the 4B5B, a pattern of 1 is four and 0 is one, 1 is three and 0 is two, and 1 is two and 0 is three, etc., are mixed in the transmitted 5-bit pattern. Therefore, when the pattern in which 1 is four and 0 is one continuously comes out, brightness is 80% but when the pattern in which 1 is two and 0 is three is continued, brightness is reduced to 40%. When the pattern is repeated, the LED lighting is flickered and the flickering is a serious problem in the visible light wireless communication using the LED lighting.